Polyolefins (for example, polyethylene and polypropylene) are melt processed above their crystalline melting points. The melt strength of the polyolefin, if too low, may cause difficulty in melt processing processes, such as thermoforming and blow molding. Low melt strength is exhibited as sag or drool.
Sag is a surrogate measurement that relates to both melt elasticity and melt strength. Good sag resistance is conducive to enhancement of thermoformability in plastics (J.L. Throne, Technology of Thermoforming, Hanser-Gardner Publications, Inc., Cincinnati Ohio, 1996). Several approaches to produce melt-strength have been developed over the years. Irradiating polypropylene to obtain long-chain branching and, consequently, high melt strength, has been commercialized. This method involves careful control of the irradiation process and may lead to the undesirable production of gels. Treatment of LLDPE with organic peroxides is another route to obtain a polyolefin with high melt strength although, as in the irradiation case, the process involves a chemical reaction that must be closely controlled to produce the desired effect.
Alternatively, additives have been used to improve melt strength, such as Mitsubishi Rayon's A-3000 which is a commercial product based on an acrylic polymer and polytetrafluoroethylene (PTFE). Chemically modified LLDPE, as is the case of DuPont's BOOSTER, is another example of the additive approach to enhance melt strength in polyolefins. Long-chain acrylic monomers as additives to improve melt strength have been used but the synthetic route to obtain such additives is cumbersome.